In wireless communication systems, each mobile station might be coupled to a communication network by associating that mobile station with a particular AP (access point). In some IEEE wireless communication standards, wireless packets may be sent unicast (to a single destination) or multicast (to more than one destination). “Broadcast” packets are that special case of multicast packets where the designated “more than one destination” includes substantially all destinations in the wireless communication system.
In some IEEE wireless communication standards, such as the IEEE 802.11 family of communication protocols, multicast packets are not reliably delivered. In wireless communication systems following an IEEE 802.11 protocol, unicast packets must be responded to by the destination, with an acknowledgement (ACK) packet. A higher-level protocol is able to ensure reliable delivery by known techniques that are employed when a message is sent to, but no ACK packet is received from, the destination.
While this technique is generally adequate to assure delivery of unicast packets, many communication protocols, including the IEEE 802.11 family of communication protocols, do not require the destination to respond with an ACK packet when it receives a multicast packet. This could be due to concern that the sender would be flooded with ACK packets, or might be due to the nature of traffic using multicast packets. For example, multicast packets are often used to broadcast streaming audio, video, or audio-video, signals to a recipient audience. In some cases, such as election news, sports events, and the first episode of the “Victoria's Secret” modeling show, that audience may be numbered in millions.
While the loss of some fraction of packets for streaming audio-video (for example) is not necessarily crippling, it can have a noticeable effect for a user at the destination. Missing even 50 milliseconds of audio, for example, can be noticed by many human ears, with the effect that loss of even a small fraction of packets may make the streaming signal seem broken-up or jumpy.